Electromagnetic stirring is a frequently employed process in metals processing operations. Representative examples include induction stirring of the mold region of continuous casters and the induction stirring of ladles in ladle metallurgy operations.
A recently suggested application of electromagnetic stirring is in the field of rheocasting or the casting of composite materials, where intensive stirring is required to impart fluidity to melt-solid suspensions. Intensive agitation is required to reduce the apparent viscosity of such systems.
Electromagnetic stirring generally involves inducing a rotating motion in a melt in a horizontal plane, or, alternatively, a predominantly vertical motion may be induced in the melt through the use of linear stirrers.
Many other stirring possibilities exist, involving different geometries, including the molds of slab, thin slab and bar casters, with the molds having vertical, horizontal or other orientation. Furthermore, the actual stirring to be employed may produce predominantly vertical, horizontal or helical motion. Stirring may be continuous, intermittent or provide alternating directions for the velocity field.
One potential problem with most prior art stirring applications is the fact that, when there exists a free surface, such as exists in continuous casting when the mold region is being stirred and also in ladle metallurgy applications, intensive stirring can distort the meniscus and may produce disturbances or waves on the free surface.
As an example of this problem, when horizontal, rotational flow is being induced in a cylindrical container, a central depression is generated, the depth of which is determined by the expression: ##EQU1## wherein: h is the depth of the depression,
w is the angular velocity, PA1 R is the radius of the cylinder, and PA1 g is the acceleration due to gravity.
The meniscus becomes distorted at the walls due to upward flow of metals and wave formation may occur. Such distortion in the meniscus shape and the formation of waves is highly undesirable in many applications of electromagnetic stirring to continuous casting.
More specifically, when mold powders are being used, which often is the case, free surface disturbances can lead to entrainment of the mold powder in the molten metal and hence the presence of impurities occluded in the finished product.
Intensive metal circulation also may lead to erosion of pouring tubes immersed in the molten metal and through which the molten metal is fed to the mold. In addition, the quite high velocities that may be desirable for certain applications, for example, rheocasting or the production of very fine grain structures, may result in unacceptably large meniscus deformations.